Direct evidence of substorm-related impulsive injections of electrons at Mercury

Mercury’s magnetosphere is known to involve fundamental processes releasing particles and energy like at Earth due to the solar wind interaction. The resulting cycle is however much faster and involves acceleration, transport, loss, and recycling of plasma. Direct experimental evidence for the roles of electrons during this cycle is however missing. Here we show that in-situ plasma observations obtained during BepiColombo’s first Mercury flyby reveal a compressed magnetosphere hosts of quasi-periodic fluctuations, including the original observation of dynamic phenomena in the post-midnight, southern magnetosphere. The energy-time dispersed electron enhancements support the occurrence of substorm-related, multiple, impulsive injections of electrons that ultimately precipitate onto its surface and induce X-ray fluorescence. These observations reveal that electron injections and subsequent energy-dependent drift now observed throughout Solar System is a universal mechanism that generates aurorae despite the differences in structure and dynamics of the planetary magnetospheres.

Line 173: Given the Hermean magnetosphere was in a compressed state, under an estimated solar wind dynamic pressure of 28 -60 nPa, would the authors be able to provide a comment on how a nominal solar wind would affect the electron magnetospheric injection and energy-dependent drift processes in the Hermean magnetosphere?

S. Aizawa et al.
This paper presents initial results for electrons from the first flyby of Mercury from the BepiColombo spacecraft. Electron energy spectra are shown during the flyby which strongly indicate that there are impulsive injections of electrons into the inner magnetosphere of Mercury. The paper shows results from the MPPE instrument, in particular the Mercury Electron Analyzer (MEA), which for the first time at Mercury can directly observe electrons in the energy range less than 30 keV. This is particularly valuable since the MESSENGER spacecraft did not have an instrument capable of making direct measurements of electrons in the eV to ~ 30 keV range, although the XRS instrument on MESSENGER was able to detect the presence of 1-10 keV electrons indirectly [e.g., Ho et al. (2016), paper (3) in the Main references]. The results presented in this manuscript provide detailed electron energy spectrograms during the flyby, which indeed show strong evidence of electron injections and energy-time dispersion, strongly supporting the notion that ULF waves and magnetic substorm behavior occur at Mercury. Overall, the manuscript is very well written, nicely organized, and includes interesting and exciting new results. I highly recommend publication after addressing some minor questions and suggestions listed below.
Introduction Main Text line 63: please add a couple of references that discuss X ray fluorescence and other consequences of electron precipitation of Mercury. A reference to Starr et al. (2012) should be included since this was the first paper to discuss electron-induced X-ray fluorescence at Mercury. It is also suggested at this point to add another consequence of electron precipitation at Mercury, i.e., electron stimulated desorption (ESD), which can result in the emission of heavy ions from the surface into the magnetosphere (Schriver et al., 2011;McLain et al., 2011). Full references for these papers are given at the end of the review.
Line 103: Since pressure is being referred to, presumably it should be 28 ~ 60 nPa (not nT).
In the section starting on line 107, Low-frequency fluctuations on the dusk flank of Mercury's magnetosphere: Is there any magnetometer data available to make correlations with the particle data? Either way, if there is or isn't, it might be worth mentioning the status of magnetometer data as it relates to these flybys and what could be done in the future. Figure 4, line 358: There is a reference to paper (8), but this should likely be referring to paper (7) by Lindsay et al. The results presented in this manuscript provide detailed electron energy spectrograms during the flyby, which indeed show strong evidence of electron injections and energy-time dispersion, strongly supporting the notion that ULF waves and magnetic substorm behavior occur at Mercury. Overall, the manuscript is very well written, nicely organized, and includes interesting and exciting new results. I highly recommend publication after addressing some minor questions and suggestions listed below.
Introduction Main Text line 63: please add a couple of references that discuss X ray fluorescence and other consequences of electron precipitation of Mercury. A reference to Starr et al. (2012) should be included since this was the first paper to discuss electroninduced X-ray fluorescence at Mercury. It is also suggested at this point to add another consequence of electron precipitation at Mercury, i.e., electron stimulated desorption (ESD), which can result in the emission of heavy ions from the surface into the magnetosphere (Schriver et al., 2011;McLain et al., 2011). Full references for these papers are given at the end of the review. è Thank you for these additional references. We have now added the three references suggested on Lines 63-64, together with an explicit mention of the impact of electron precipitation on the emissions of heavy ions from the surface into the magnetosphere.
Line 103: Since pressure is being referred to, presumably it should be 28 ~ 60 nPa (not nT).
In the section starting on line 107, Low-frequency fluctuations on the dusk flank of Mercury's magnetosphere: Is there any magnetometer data available to make correlations with the particle data? Either way, if there is or isn't, it might be worth mentioning the status of magnetometer data as it relates to these flybys and what could be done in the future. è Magnetometer data are not yet available. The instrument team is hardly working on removing disturbances and offsets on their measurements. Thus, we did not discuss the status of magnetometer data in the manuscript. However, a joint investigation between our plasma data and data from the magnetometer is expected in the future. We have clarified it on lines 121-122. -> Without magnetic field data, it is indeed not easy to identify the magnetospheric modes that we have observed. The first low-frequency oscillations reported in the manuscript were observed during the inbound leg of BepiColombo's trajectory when the magnetospheric boundaries were observed to be closed to their average locations, i.e. not compressed. The solar wind conditions have changed during the outbound leg of BepiColombo's trajectory as shown in Figure 2. Therefore, our interpretation in terms of field line resonance for the first low-frequency oscillations reported cannot be ruled out. We have now added a reference to James et al., 2019 on line 118 to provide context observations with previous studies.
The authors sincerely thank the reviewers for reviewing our manuscript again and we acknowledge the comments received from both reviewers. We have modified our manuscript accordingly.
Reviewer #1 ( 2) In addition, reference (15) è LLBL is a region where both magnetosheath plasmas and magnetospheric plasmas exist. In the LLBL plasmas have higher temperature and density that gradually when moving closer to the inner magnetosphere, as shown in Lockwood and Hapgood (1997) for instance. In our opinion it is difficult to firmly conclude to the crossing of the LLBL from the SERENA observations since the PICAM data displayed in their paper are not given in physical parameters (only raw counts) and the raw data shown in their Figure   The authors have addressed all issues and comments raised in both referee reports satisfactorily and publication should proceed as soon as possible. è Thank you very much for reviewing our manuscript.